Recommendation R.20
Replace by the following:
TELEGRAPH MODEM FOR SUBSCRIBER LINES
(Geneva, 1980; amended at Malaga-Torremolinos, 1984
and at Melbourne, 1988)
The CCITT,
considering
(a) that the use of high-level telegraph transmission with single or
double current may cause disturbing impulse noise in adjacent cable pairs that may
be eliminated by applying low-level transmission with telegraph modems;
(b) that telegraph modems would substantially reduce the power
consumption in the central office;
(c) that where connection to a subscriber has to be achieved over a non-
metallic pair (e.g., a voice-channel frequency-division multiplex or pulse code
modulation system) a telegraph modem has to be utilized;
(d) that the frequencies given below are already standardized in
Recommendation V.21 [1];
(e) that suitable inexpensive telegraph modems can be used for full
duplex transmission on 2-wire circuits at modulation rates up to 300 bauds;
unanimously declares the view
that where low-level telegraph transmission is used, the following method
of transmission should be recommended for all modulation rates up to 300 bauds.
1. Channel allocation
The method of transmission is based on Recommendation V.21 [1] with the
following frequency designations:
Central office to subscriber (channel 1) FA = 1 180 Hz,
FZ = 980 Hz;
Subscriber to central office (channel 2) FA = 1 850 Hz,
FZ = 1 650 Hz.
The characteristic frequencies as measured at the telegraph modem line
output should not differ by more than + 3 Hz from the nominal figures.
The modem shall continue to operate with a + 6 Hz change in receive
frequency.
It should be noted that there is equipment in use that applies alternative
frequencies to those shown in this Recommendation.
2. Interface
Where the modem is a separate, self-contained unit, the following
interchange circuits shall be used:
Common return (e.g. circuit 102 in Recommendation V.24 [2])
Transmitted data (e.g. circuit 103 in Recommendation V.24 [2])
Received data (e.g. circuit 104 in Recommendation V.24 [2])
Carrier detect (e.g. circuit 109 in Recommendation V.24 [2]).1
3. Electrical characteristics
The electrical characteristics (for stand alone telegraph modems) of the
interchange circuits, should be in accordance with CCITT Recommendation V.28 [3].
4. Performance
4.1 The modem under test shall be connected to another modem (according to
this Recommendation or to Recommendation V.21) via an attenuator having a return
loss of 4 dB and an insertion loss of 30 dB. The relative group-delay
characteristics of the transmit filters are subject to further study.
4.2 Uniform spectrum Gaussian noise (band limited to 10 kHz) shall be added to
give a normalized signal-to-noise ratio of 32 dB.
This is defined as:
SIGNAL ENERGY PER BIT = SIGNAL POWER x NOISE BANDWIDTH = 32 dB
NOISE POWER PER HERTZ NOISE POWER SIGNAL BIT RATE
1 This is considered optional, particularly at the subscriber's location.
4.3 Test signals to Recommendation R.51bis (QKS) shall be sent at a -13 dBm
transmit level in both directions simultaneously. (To ensure incoherence, the rate
of the test signals for the direction not under test shall be slightly lower.) The
test period shall be 15 seconds.
The performance shall be in accordance with Table 1/R.20.
TABLE 1/R.20
4.4 In certain configurations, it is not possible to check the modem
distortion, e.g. telegraph terminals, multiplex and switching equipment with
integral telegraph modems on which the d.c. signal output of the telegraph modem
is inaccessible for any reason. The performance will normally be in the form of a
test for distortion margin before errors are output from the equipment.
Note - The modem need only be tested at the maximum modulation rate of the
equipment into which it is to be incorporated.
4.5 The design of the telegraph modem shall be such that the transmission
performance is guaranteed without adjustment on installation or subsequently.
5. Line signal levels and terminating impedance
The line interface of the modem shall be balanced and shall present an
impedance of 600 ohms with a return loss (reference 600 ohms resistive) of not
less than 14 dB (reflection coefficient no greater than 20%) over the range
300 - 3 400 Hz.
5.1 Output signal level
5.1.1 When terminated in 600 ohms the output level should be set to -13 dBm
Note - In certain applications, in particular if carrier circuits for voice
channel frequency division multiplex or pulse code modulation systems are used
(where the input level should be limited to -13 dBmO) it may be necessary to have
a setting range of output level which could give up to 0 dBm.
5.1.2 The difference in output levels between the Binary 1 (condition Z) and
Binary 0 (condition A) signals shall not be greater than 1 dB for either of the
channels.
5.2 Receive signal level
5.2.1 When the received signal level is -43 dBm or greater, the equipment should
correctly interpret the line condition as either Fa or Fz.
5.2.2 When the received signal level remains below a threshold between
-45 dBm and -48 dBm, the equipment shall ignore incoming information within 300 ms
of the signal level dropping below the threshold. During this delay the receiving
equipment may interpret (correctly or incorrectly) characters received after the
drop in received signal level below the threshold. Thereafter, the equipment
should not interpret characters received until the received signal level has
restored to at least 2 dB above the threshold. If the signal level drops below the
threshold for less than 10 ms, the equipment should take no action.
5.2.3 Once the received signal level has fallen below the threshold value, the
equipment shall ignore any subsequent increases in the signal level which are
either less than 2 dB above the threshold or of more than 2 dB above the threshold
but less than 10 ms in duration.
Note 1- The requirements of 5.2.3 need not apply during the first 20 ms from the fall
in received signal level below the threshold.
Note 2 - The signal level detector shall respond to the total power contained within
the nominal spectrum of the received line signal.
6. Maintenance facilities
Maintenance facilities, e.g. test loops, are a national matter.
7. Protection against high voltages
The equipment shall withstand residual lightning surges, high voltage line
faults, and high level telegraph keying. The protection required is considered a
national matter, though in some cases Recommendation K.y could be applied [3].
8. Line wetting
8.1 Line wetting, where required by the type of line plant used, will normally be
sourced by the in-station equipment and looped by the subscriber's equipment.
8.2 The current has a maximum value of 15 mA with the line short-circuited. The
wetting current shall be a minimum of 5 mA on 4000 ohms lines. The open circuit
voltage shall be under 80 V.
Note - In some countries, other values may apply.
Where a modem may be required to work in the presence of line wetting, then
the modem performance ( 4) and the modem line impedance ( 5) requirements must apply
with line wetting current flowing.
The noise applied to the line from the wetting power supply shall be under -
80 dBm (600 ohms) over a range 300 to 3 400 Hz (flat).
9. Permitted out-of band energy
9.1 The out-of-band energy is a national matter; however, the following
information is provided to assist equipment manufacturers.
9.2 The transmitted signal level (with an output level at -13 dBm) at any one
frequency shall not exceed the following limits (with the forward and reverse channel
keyed at 300 bit/s with a QKS code):
200 Hz to 3 200 Hz -13 dBm
100 Hz to 200 Hz and 3 200 Hz to 3 400 Hz -23 dBm
Below 100 Hz -33 dBm
Above 3 400 Hz -33 dBm and decreasing
by 12 dB/octave to
-67 dBm.
REFERENCES
[1] CCITT Recommendation - 300 bit/s per second duplex modem standardized for
use in the general switched telephone network, Recommendation V.21.
[2] CCITT Recommendation - List of definitions for interchange circuits between
data terminal equipment adn data circuit terminating equipment,
Recommendation V.24.
[3] CCITT Recommendation - Electrical characteristics for unbalances double-
current interchange circuits, Recommendation V.28,
[4] CCITT Recommendation - Resistibility of subscribers' terminals to over-
voltages and over-currents, Recommendation K.y.